Question

What do you understand by short and long periods of periodic table ?

Hint:

To easily recall the number of elements per period, remember the sequence: 2, 8, 8, 18, 18, 32, 32. This corresponds to periods 1 through 7. Periods 1-3 are short, 4-5 are long, and 6-7 are very long.

Answer 1

The horizontal rows of the modern periodic table are called periods. They are classified as short, long, and very long based on the number of elements they contain, which relates to the filling of electron shells.
\[\begin{array}{rl} \bullet & \text{Short Periods:} \\ \bullet & \text{1st Period: Contains only 2 elements (Hydrogen, Helium). It is the shortest period, corresponding to the filling of the first energy shell (n=1).} \\ \bullet & \text{2nd and 3rd Periods: Each contains 8 elements. These are also considered short periods. They correspond to the filling of the second (n=2) and third (n=3) energy shells, respectively.} \\ \end{array}\] \item Long Periods: \[\begin{array}{rl} \bullet & \text{4th and 5th Periods: Each contains 18 elements. These are called long periods. This is because, in addition to the 's' and 'p' orbitals, the 'd' orbitals of the penultimate shell also start filling, which accommodates 10 additional elements (the transition metals).} \\ \end{array}\] \item Very Long Periods: \[\begin{array}{rl} \bullet & \text{6th and 7th Periods: Each contains 32 elements. These are the longest periods. In these periods, the 'f' orbitals of the ante-penultimate (second to last) shell are also filled, accommodating 14 more elements (the lanthanides in period 6 and actinides in period 7), which are usually placed separately below the main table.} \\ \end{array}\] \end{itemize}